It depends. Under normal circumstances I would say yes. The question though is whether or not you need to collect your luggage which if you are flying United then that won't be a concern but if you travel between the terminals you will have to go through security. And depending on when your flight is it may be pretty packed.

So once you arrive at terminal A you should go up to the Airtran from Terminal A to Terminal C. The entire ride takes all of about 3 minutes and trains run every 3-5 minutes at that time you should go down to the security and go through the security checkpoints that are on sides of the terminal I think those are checkpoints 1 and 3. They are usually less loaded then the central one (2).